The present invention relates generally to the field of borehole gravity gradiometers. Specifically an apparatus for measuring borehole gravity gradient is provided with means for selectively tuning the gravity gradient detection sensitivity of the apparatus.
Variations in the gravitational field and gravity gradient can provide an estimate of various subterranean formations. Additionally, gravity gradient measurements from a borehole are of interest to exploration geophysicists due to their simple, direct relationship with the subterranean formation density surrounding the borehole.
Presently, a variety of apparatuses are available to measure borehole gravity gradients. First, a borehole gravity meter, e.g., LaCoste-Romberg, as described by A. R. Brown and T. V. Lautzenhiser, Geophysics, v. 47, January, 1982, p. 25-30, can be employed to measure gravity at selected depths within a borehole from which an average gravity gradient can be computed. Second, a borehole gravity gradient can be obtained directly from a suspended dipole mass gravity gradiometer.
Looking to FIG. 1, a suspended dipole mass gravity gradiometer A comprising a dipole mass system D having a pair of symmetrically distributed masses m attached to a beam B is shown. The beam B is suspended from its midpoint with a suspension ribbon R such that the beam B is free to rotate about a horizontal axis, e.g., x or y axis. A gravity gradient, for example, ##EQU1## can produce a measurable torque N.sub.g and angular displacement .theta. of the dipole mass system D from a reference or equilibrium position can be sensed and recorded.
The angular displacement .theta. of the dipole mass system D from its equilibrium or reference position is proportional to the square of the period of the natural oscillation for the dipole mass system D about the rotational axis and the torque N.sub.g applied to the dipole masses m by the gravity gradient G.sub.zx. Hence, the sensitivity of the suspended dipole mass gravity gradiometer A can be characterized by its natural period.
Since the dipole mass system D is suspended by a fine metallic ribbon R, a resistive torque N.sub.r is developed due to the resistance of the metallic ribbon R to flexure or bending. As such, the actual sensitivity of the suspended dipole mass gravity gradiometer A is significantly reduced because the torque required to bend metallic ribbon R tends to reduce the angle of displacement .theta. from the equilibrium or reference position.
One way to decrease the resistive torque N.sub.r for bending the metallic ribbon R is to reduce the cross-sectional area of metallic ribbon R, i.e., a thinner ribbon. However, this also has the effect of decreasing the load-bearing capabilities of the metallic ribbon R and as well as the moment of inertia of the dipole mass system D which is counterproductive to increasing the sensitivity of the gravity gradiometer A.